To some, it is obvious that Intelligent Design is not science, yet it may be difficult to articulate exactly why that is the case. This essay hopefully will help with that problem.

Intelligent Design tries to do a few things. First, it wants to change generally accepted philosophical principles behind science where it disagrees with them. Secondly, and this is key, it wants to add philosophical assumptions of its own. Third, regarding other philosophical principles, it uses rhetoric to try to distort the way they are understood. Fourth, it points out features of evolution that science does not yet understand. If presented fairly, there is nothing wrong with this fourth point, in my opinion. But, they do not attempt an unbiased presentation, and instead use rhetoric favorable to their ideas. Here we’ll just look at the first three of these points.

First we need a definition of science. Unfortunately there is no one universally accepted definition. However, some things are widely accepted. A good definition should not just describe science, but also tell us what makes it unique. The definition I favor here does that. It is also substantially the same as the one proposed by Ian G. Barbour in his book, "Religion and Science: Historical and Contemporary Issues".
1) Science must rely only on data that is public and/or reproducible. Other forms of evidence may be perfectly valid in other human endeavors, for example testimony in law. But science must restrict itself to verifiable data inputs.
2) Science should make no other assumptions beyond the assumptions it needs in order for inductive and deductive logic to operate. That is - science should induce things from data, and make logical deductions to demonstrate what is probable about nature, but make no assumptions beyond that.
3) Other things we could say about science may be useful in describing it, but they will not tell us what makes science unique.

So, how does Intelligent Design stack up? Actually, so far, O.K. Creationism would make added assumptions and use non-verifiable data, but it’s not clear that Intelligent Design does this. So let’s look a little closer. Is “design” even a valid type of scientific explanation? Let’s look at an example. If I place a book on the shelf, one explanation of how the book got there is that I intended to put the book there. But that would not be the answer science is generally looking for. It wants to know what happened in terms of muscle cells and neurons. That is – it is reductionistic. It does not stop with a high level explanation, but tries to dig deeper. “Design” would be the most complete scientific explanation, only if we could not reduce it any more than that. So far, so good. Intelligent Design advocates would claim that they just want to present evidence of irreducible design.

Philosophical materialism would claim that “design” does not even exist as a category. On the other hand, traditional monotheistic religions are generally philosophically Dualistic. That is - they believe that mind and matter are two separate categories of things. This is also known as “Cartesian Dualism”, after Rene Descartes. In this view design and natural material causes are contradictory explanations. Design implies a non-material mind in Dualistic philosophy. Other philosophical paradigms may admit design as a valid category of explanation, but not see it as contradictory to natural causes. In the field of artificial intelligence this would be related to a “functionalist” view. Process philosophy also uses this philosophical paradigm. Also, a number of modern books take up the subject of “emergence”. Although these may be partly Dualistic in their own way, or not, depending on the approach of the author. And, finally, many Eastern religions adopt this philosophy. As an example, in this sort of "non-contradictory" paradigm one could believe that one made a choice to place the book on the shelf, and at the same time the action is ultimately explicable in terms of neurons, muscles, brain activity, electrical signals, etc. In this paradigm, these are not contradictory explanations. Neither would evolution and "design" be viewed as contradictory explanations. Science is generally said to engage in “methodological naturalism”. That is - it makes no specific judgments about the validity of design as an explanation, or mind as a separate category, but following its reductionist program, it tries to find what natural causes it can. So, what Intelligent Design wishes to do is to have science abandon “methodological naturalism” and replace it with a philosophy of Dualism.
This last part is left unstated, however.

Even if we allow that “design” should be valid type of scientific explanation, stopping with that would not be widely accepted as science. And most importantly, without the assumption of Dualism evidence of design is not evidence against evolution. By presenting evidence of design as evidence that explicitly contradicts evolution, Intelligent Design advocates are implicitly advocating an unstated philosophy of Dualism, a basis of traditional monotheistic religion.
As a side note here, monotheism can be maintained without an assumption of dualism, but that has not been the case traditionally.

In other cases advocates of Intelligent Design distort the understanding of underlying principles. For example the inputs of science must be verifiable. But science uses these inputs to make predictions about what it can not observe. That is what every scientific theory does. Intelligent design claims that “microevolution” is valid, but because we have not observed “macroevolution” that is not valid, or is only a “theory”.

Here we need to digress, and say that everything science does is “only” a theory. The term “Law” is something left over from the enlightenment period of science. Things in science never get promoted to something beyond the level of “theory”. Science does not prove things absolutely true, but rather it demonstrates that some things are highly probable.

The raw, verifiable data in evolution is in the fossils and the DNA and structure of creatures we can observe, and have observed in the past. “Microevolution” then is part of the verifiable inputs to the process of science. “Macroevolution” is the theory that emerges from the process of science as a result of logical induction and deduction. That is - “macroevolution” is what the scientific method shows to be the most probable explanation, or the best theory for the history of life on earth.

This need not contradict Theism, however. Evolution requires both natural selection and random mutation. For "random" the Theist could read "non-deterministic" and then see at least some non-deterministic events as choices made by God. According to science, evolution has the capacity to produce the observed changes over time we see in the fossil and DNA record. According to the Theist, God also has the capacity to cause these changes. Thus the Theist can attribute the evolution of humans on earth to choices made by God, or to some combination of choices and random events.Conflict eliminated.

This is not the approach taken by "intelligent design" however. Intelligent design uses arguments like "irreducible complexity" to try to say that evolution does not have the capacity to have produced the observed changes we see. Irreducible complexity involves the idea that certain structures could not have evolved, because they are multi-part, and the parts are useless alone. The answer to this involves scaffolding. In human constructions scaffolding may be used to assist in construction, and removed later. This same sort of process can lead to “irreducible complexity” in living things. http://www.talkorigins.org/faqs/behe.html

Society can produce things that appear to be irreducibly complex too. There would be no need to build a gasoline refinery without cars, but no one would build cars without gasoline. If we don't think that both could come into being at the same time, then which came first and why? In this case gasoline was first produced as a by-product of kerosene for lamps. Anyone who has seen the show "Connections" knows that society can develop something for one purpose, and later use it for something else. The evidence of its original purpose may then disappear. There is no reason evolution can not do the same thing.

The simplest way to look at this is just to say that we have to remember that evolution can remove parts as well as add them. "Irreducible complexity" tries to argue that the structures could not be built one piece at a time. But even if that is true, it does not eliminate the possibility that something larger could have been built one step at a time, and afterwards pieces were removed.

In closing I just want to reemphasize the most important points. Intelligent Design carries with it, implicit in its very premise, the philosophical assumption of Dualism. Design and evolution are only contradictory explanations under the philosophy of Dualism, and the very premise of Intelligent Design is that evidence of design is evidence against evolution. Critics of Intelligent Design claim it is religion in disguise. To which advocates respond, that they have never mentioned religion. Rather critics should point out that Intelligent Design advocates a philosophy of Cartesian Dualism, which is a core philosophical assumption of traditional monotheistic religions.

Theism and evolution can exist together by just replacing the word "random" with "non-deterministic", but "intelligent design" does not do this, rather it uses rhetoric and the name of science to try to claim that evolution does not even have the capacity to have caused what we see.